1. Field of the Invention
The present invention relates to a surface acoustic wave (SAW) device having a balanced-to-unbalanced conversion function, and a communication apparatus including such a SAW device.
2. Description of the Related Art
There has been remarkable technological progress in the reduction in the size and weight of communication apparatuses in recent years. In order to achieve such reduction, not only are components reduced in number and size, but also combined components formed by combining functions have been developed.
With these circumstances as background, a SAW device for use in a radio frequency stage of a cellular phone having a balanced-to-unbalanced conversion function, or a so-called “balun” function, has been actively studied in recent years. Mainly, global systems for mobile communications (GSM) have come into use. Some patent applications concerning SAW devices provided with balanced-to-unbalanced conversion functions as described above have been filed.
In addition, recently developed cellular phone systems have close transmitting and receiving frequencies which have increased. With such increase, an attenuation in a receiving frequency band must be increased in a transmitting filter, and an attenuation in a transmitting frequency band must be increased in a receiving filter. Accordingly, in a SAW device, the need to increase an attenuation in the vicinity of a passband has become strong.
In a communication apparatus, it is common to use a characteristic impedance of 50 Ω in a portion from an antenna to a filter for an unbalanced signal. In many cases, in an amplifier or other component which is used after the stage of the filter, an impedance of 150 Ω to 200 Ω is used for a balanced signal.
Regarding a SAW filter device (SAW device) which also has a function of converting a 50-Ω unbalanced signal into a balanced signal having 150 Ω to 200 Ω, for example, a device in which four SAW filter elements are used to realize an unbalanced input and a balanced output is known (see, for example, Japanese Unexamined Patent Application Publication No. 10-117123). The configuration of the SAW filter device disclosed in this publication is shown in FIG. 24.
In the configuration, which is known, a spurious component in the vicinity of the passband outside of the passband is caused by excitation of surface acoustic waves. In a range in which the spurious component is generated, the phase characteristics of balanced terminals 506 and 507 are inverted as in the passband. Accordingly, a signal canceling effect in a differential state is not obtained, thus causing a problem in that the attenuation in the vicinity of the passband outside the passband is insufficient.
SAW device configurations of the related art are described below with reference to FIGS. 19 to 24.
FIG. 19 shows a SAW device 100 in which, in a configuration having the impedances of balanced signal terminals 114 and 115 which differ by approximately four times the impedance of an unbalanced signal terminal 113, an attenuation in the lower range of a passband can be increased.
As shown in FIG. 19, the SAW device 100 includes two longitudinally-coupled-resonator SAW filters (SAW filter elements) 101 and 102 on a piezoelectric substrate (not shown). The longitudinally-coupled-resonator SAW filter 101 includes three interdigital transducers (IDTs) 103, 104, and 105, and reflectors 106 and 107 on the sides of the IDTs 103 and 105. Similarly, the longitudinally-coupled-resonator SAW filter 102 includes three IDTs 108, 109, and 110, and reflectors 111 and 112 on the sides of the IDTs 108 and 110. The IDTs 103 to 105, and the IDTs 108 to 110 are arranged in a row along a propagation direction of surface acoustic waves. In the SAW device 100, the IDTs 108 and 111 of the longitudinally-coupled-resonator SAW filter 102 are inverted along an interdigital length compared with the IDTs 103 and 105 of the longitudinally-coupled-resonator SAW filter 101. In this arrangement, in the longitudinally-coupled-resonator SAW filter 102, the phase of an output signal to an input signal differs by approximately 180 degrees compared with the longitudinally-coupled-resonator SAW filter 101.
The IDTs 104 and 109 are connected to a signal terminal 113. The IDTs 103 and 105 are connected to a signal terminal 114. The IDTs 108 and 110 are connected to a signal terminal 115. In the SAW device 100, the signal terminal 113 is an unbalanced signal terminal, and the signal terminals 114 and 115 are balanced signal terminals.
Each IDT includes two electrode-finger units each including a bar base-end portion (bus bar) and a plurality of mutually-parallel bar electrode fingers which extend in a perpendicular direction from one side of the base-end portion. Both electrode-finger units are arranged so that the side portions of the electrode fingers of one unit face the side portions of the electrode fingers of the other unit. Also, each reflector is used to reflect transmitted surface acoustic waves. The reflector includes a pair of bar base-end portions (bus bars), and a plurality of mutually-parallel bar electrode fingers which extend in a perpendicular direction from one side of one base-end portion and which electrically connect the base-end portions.
As described above, the SAW device 100 which has a balanced-to-unbalanced conversion function and in which the impedance of balanced signal terminals differs by approximately four times that of an unbalanced signal terminal is realized.
A feature of the SAW device 100 is that the number of electrode fingers in each of the reflectors 111 and 112 is less than that in each of the reflectors 106 and 107. In other words, the longitudinally-coupled-resonator SAW filters 101 and 102 have different numbers of electrode fingers.
As a comparison with the SAW device 100, a SAW device 120 (comparative example 1) which includes two longitudinally-coupled-resonator SAW filters 101′ and 102 on a piezoelectric substrate (not shown) is shown in FIG. 20. The SAW device 120 has a configuration in which, in the SAW device 100, instead of the reflectors 106 and 107, reflectors 106′ and 107′ which are identical in number of electrode fingers to the reflectors 111 and 112 are provided. In other words, in the SAW device 120, the longitudinally-coupled-resonator SAW filters 101′ and 102 have the same number of electrode fingers of reflector. The other points in configuration are identical to those in the SAW device 100.
FIG. 21 shows frequency-to-insertion-loss characteristics of the SAW device 100 shown in FIG. 19 and the SAW device 120 shown in FIG. 20. From FIG. 21, it is seen that the configuration of the SAW device 100 has an improvement of approximately 5 dB around 1780 MHz in the passband compared with the SAW device 120. This is an effect of the difference in reflection characteristics of reflector between the longitudinally-coupled-resonator SAW filters 101 and 102.
The configuration of the SAW device 100 in FIG. 19 has a problem in that an attenuation in identical components (common mode) in the lower range of the passband deteriorates. FIG. 22 shows frequency-to-common-mode-attenuation characteristics of the SAW device 100 and SAW device 120. A common mode attenuation around 1640 MHz to 1780 MHz is 30 dB in the SAW device 120, while it deteriorates to 23 dB in the SAW device 100.
This is caused by deterioration in amplitude balance and phase balance in the lower range of the passband due to the difference in reflection characteristics of reflector between the longitudinally-coupled-resonator SAW filters 101 and 102. For recent SAW devices having a balanced-to-unbalanced conversion function, it is required that an attenuation be increased not only in the passband, but also in the common mode outside the passband. The configuration of the SAW device 100 has a technical problem in that the request cannot be satisfied.